1. Field of the Invention
This invention relates to the measurement of the flow of molten aluminum, and in particular, the flow of molten aluminum through a trough using a magnetic flow meter.
2. Background Information
Magnetic flow meters for measuring the flow rate of electrically conductive liquids are well known. They operate on the principle that a conductive liquid flowing through a magnetic field applied at right angles to the direction of flow generates an electromotive force or voltage. This voltage is proportional to the velocity of the conductive liquid. By knowing the cross-sectional area of the flow, the volumetric flow rate is easily calculated.
Magnetic flow meters have some very desirable characteristics. First, the design is very simple; there are no moving parts. There also are no internal impediments to flow, they can operate in high temperature environments, they are versatile in that they can be installed in any desired location or position, they have a broad measurement span, they can measure bidirectional flow, they are unaffected by variations in viscosity, pressure, temperature and density of the media, and they have an accuracy of at least better than plus or minus 2%.
Magnetic flow meters have been successfully employed in industry for over two decades for monitoring flow control of molten sodium in nuclear reactors and for the measurement of the flow rates of molten brass, lead and a variety of other fluids with the only criteria for implementation being that the media be electrically conductive.
However, applicants are aware of no successful application of a magnetic flow meter to measuring the flow of molten aluminum, which presents special problems. First, the temperature of the molten aluminum is high, typically about 1350.degree. F. Also, molten aluminum is highly reactive. Not only must the components which contain the molten aluminum be resistant to corrosion, but even more importantly, the electrodes of the magnetic flow meter must be capable of operating with hot molten aluminum for prolonged periods of time without eroding, fouling or changing their electrical characteristics.
U.S. Pat. No. 4,741,216 proposes either lining or constructing the measuring tube of a magnetic flow meter through which molten aluminum passes with silica carbide to resist corrosion. However, this patent also suggests using wire electrodes without specifying any particular material. Common high temperature electrode materials like steel wire are rapidly dissolved by molten aluminum. But even slow erosion causes drifting of the meter as the voltage generated by the meter is directly proportional to the distance between the electrodes. Erosion or fouling changes this critical distance.
U.S. Pat. Nos. 4,507,975 and 4,716,649 disclose a magnetic flow meter for measuring the flow rate of the liquid sodium, for instance, in a nuclear reactor, and uses platinum or platinum alloys for the electrodes. Such materials are dissolved by molten aluminum.
Molten aluminum is also very difficult to contain. It can leak through any crack or crevice through which water can pass. This property of molten aluminum, considered in addition to its high temperature and reactivity, make it very difficult to select materials and effect seals.
Because of the difficulty in containing molten aluminum, it is usually conveyed in open troughs rather than enclosed pipes. In the typical production process, flow rates of a molten aluminum are not measured, but obtained indirectly by some other measurable quantity or variable of the process such as pumping speed, changes in metal height, mass flow by temperature balance calculation and casting rates. None of these approaches provide an accurate, real time indication of flow rates of the molten metal. In addition, the accuracy of each measurement is restricted to the techniques used to obtain the values of the described process parameters. At present, no direct reading operational flow meter system exists for molten aluminum flow measurement.
Accordingly, it is a primary object of the invention to provide on-line, real time magnetic flow meter measurement of molten aluminum flow rates and in particular, measurement of molten aluminum flow rates through a trough.